Headlight dimmer using field effect transistor



Feb. 7, 1967 C. E. ATKINS ETAL HEADLIGHT DIMMER USING FIELD EFFECTTRANSISTOR Filed Feb. 21, 1964 L2? ark/B (Aura/2m r/c) #3 (ave/waINVENTORS C/PQL E. flrK/A/s E0552 7 A Z/a/(m/skv BY 9km WWW ATTORNEYSUnited States Patent ()fiice 3,303,346 Patented Feb. 7, 1967 3,303,346HEADLHGHT DIMMER USING FIELD EFFECT TRANSISTGR Carl E. Atkins,Montclair, and Robert L. Ziolkowski, South Plainfield, N..l., assignorsto Tang-Sol Electric line, a corporation of Delaware Filed Feb. 21,1964, Ser. No. 346,527 5 Claims. (6i. 25ll214) The present inventionrelates to the automatic switching of automobile headlamps in responseto light from the headlamps of other motor vehicles as they approach andpass and, for this purpose, provides a non-oscillatory circuit which isvery reliable, compact, inexpensive, and simple and which will operatedirectly off an automobile battery.

This circuit employs a field effect transistor whose gate is back biasedwith respect to its source by a circuit including a photosensitiveresistive element positioned in an automobile to pick-up light from theheadlamps of oncoming motor vehicles. When the light from the headlampsof an approaching motor vehicle strikes the photosensitive resistiveelement and thereby decreases the resistance of the photosensitiveresistive element, the magnitude of the back bias on the gate isincreased there by decreasing the current flow through the field effecttransistor. When the motor vehicle passes, the increase in resistance ofthe photosensitive resistive element, due to the drop in the intensityof the light reaching the photosensitive resistive element decreases theback bias on the gate of the field effect transistor thereby increasingcurrent flow through the field eifect transistor. This variation incurrent flow through the field effect transistor with changes in lightintensity is used to control the energization and deenergization of arelay which in turn controls the flow of current through the filamentsof the headlamps so as to cause the headlamps to dim upon the approachof the motor vehicle and to brighten upon the passing of the motorvehicle.

In the preferred embodiment the photosensitive resistor is provided withan electromagnetic shutter. When the headlamps are on, this shutterpermits light to reach the photosensitive resistive element to switchthe headlamps as outlined above. However, when the headlamps are turnedoff the shutter shields the photosensitive resistive element from light,protecting the photosensitive resistive element from deterioration dueto the adverse effect of sunlight.

For a better understanding of our invention and other advantages thereofreference should be had to the accompanying schematic diagram of thepreferred embodiment.

In the schematic diagram, the high beam filaments of the headlamps ofthe automobile are illustrated by the two filaments 10 and 12, the lowbeam filaments of the headlamps are illustrated by the two filaments 14and 16, and the headlamp override foot switch of the automobile isillustrated by the switch 18. With the foot switch 18 in the high beamposition, current flows directly from the positive terminal 20 of thebattery through the foot switch 18 and the high beam filaments 10 and 12to the negative, or grounded, terminal 22 of the battery to light thehigh beams.

With the foot switch 18 in its low or high beam automatic position, theposition of the armature 26 of the relay 24 will determine whether thehigh or low beam filaments are energized. With the relay 24 deenergized,the armature is positioned against a normally closed contact 28 andcurrent flows from the positive terminal 20 of the battery through thefoot switch 18, the armature 26, the normally closed contact 28, and thelow beam filaments 14 and 16 to the negative terminal 22 of the batteryto light the low beam filaments. With the relay energized, the arma-tureis positioned against a normally open contact 29 and current flows fromthe positive terminal of the battery through the foot switch 18, thearmature 26, the normally open contact 29, and the high beam filaments10 and 12 to the negative terminal 22 of the battery to light the highbeam filaments.

The coil 30 of the relay 24 is connected between the positive terminal20 and the emitter of a PNP transistor 32. The collector of the PNPtransistor 32 is connected directly to the negative terminal 22. Thebase of the PNP transistor 32 is connected directly to the collector ofNPN transistor 36 whose emitter is connected directly to the negativeterminal 22 and through a biasing resistor 34 to the positive terminal20. Therefore, the energization of the relay 30 will depend on thevoltage drop across biasing resistor 34 which in turn depends on thecurrent flow through the NPN transistor 36.

In accordance with the present invention, current flow I through the NPNtransistor 36 is controlled by a circuit including a field effecttransistor 38 and a photosensitive resistive element 40. The sourceterminal of the field effect transistor 38 is connected to a sliding tapon a resistor 42 which is connected in series with another I resistor 44between the positive and negative terminals H 20 through resistor 42 andits sliding tap, the source to drain path of the field effecttransistor, and the thermistor 45 to the grounded terminal 22 therebyproviding a voltage drop across the thermistor 45 to control the currentflow through the NPN transistor 36.

The magnitude of the voltage drop across thermistor 45 depends on thebiasing circuit for the gate of the field effect transistor. Thisbiasing circuit includes the photosensitive resistive element 40 whichis connected between the gate of the field effect transistor 38 and thepositive terminal 20 and a resistor 46 which is connected between thegate of the field effect transistor and the negative terminal 22. Italso includes, a fixed resistor 48 and a variable resistor 50 connectedin series between the gate of the field effect transistor 38 and thenegative terminal 22 with a second armature 52 of the relay 24 and thenormally open contact 54 of that armature.

With no light impinging on the photosensitive resistive element 40, therelay 24 is kept energized by the back bias on the gate of the fieldeffect transistor provided by the above-described biasing circuit.Therefore, if the foot switch is in its automatic position, the highbeam filaments 1t) and 12 are thereby energized. However, when a motorvehicle approaches, the light from its headlamps is picked up by a lens56 positioned in front of the photosensitive resistive element 40 and istransmitted through the aperture 58 in an electromagnetically controlledshutter to the photosensitive resistive element 46. The light impingingon the photosensitive resistive element 40 causes it to decrease inresistance. This decrease in resistance causes an increase in themagnitude of the back bias on the gate of the field effect transistor 38thereby decreasing the current flow through the field effect transistor.The decrease in current flow through the field effect transistor 38drops the potential across the thermistor 45 which in turn cuts thecurrent flow through the NPN transistor 36. A drop in current flowthrough NPN transistor 36 decreases the potential across the resistor 34reducing the magnitude of the current through transistor 32 and the coil30 of the relay 24 causing the relay to become deenergized.

With deenergization of the relay, the circuit through the high beamfilaments and 12 is broken by the armature 26, and the circuit throughthe low beam filaments 14 and 16 is completed by the armature 26 therebydimming the headlamps.

The deenergization of the relay also breaks the ground connection to thebase of the field effect transistor 38 through resistors 48 and 50,removing these resistors from the biasing circuit for the field effecttransistor 38. This further increases the back bias on the gate of thefield effect transistor 38 assuring that the relay will not bereenergized by the dimming of the approaching motor vehicles headlamps.Of course, when the vehicles pass each other, the decrease in lightintensity will be sufficient to cause the relay 24 to be reenergizedturning on the high beams 10 and 12 and restoring the ground circuitthrough the armature 52.

The taps on resistors 42 and 50 permit adjustment of the circuits forvariations in sensitivity from circuit to circuit. The tap on resistor42 allows adjustment of the light intensity level at which the highbeams will go on and the tap on resistor 50 permits adjustment of thelight intensity level at which the headlamps will be switched to the lowbeams. The thermistor 45 is employed instead of a normal resistor tocompensate for variations in the characteristics of circuit elementswith temperature. To prevent the NPN transistor from being burnt out, adiode 62 is connected across the coil of the relay 30 to shunt currenttransients.

In certain cases it is desirable that the switching of headlamps not beautomatically controlled. For this purpose a switch 64 is connectedbetween the coil 30 of the relay and the emitter of the PNP transistor32. When this switch is open it prevents energization of the coil of therelay therefore leaving the low beam filaments energized unless the footswitch 18 is changed to its high beam position.

The circuit as shown at 66, is fused and is provided with a panel switch70 for extinguishing the lights altogether. When the lights areextinguished, the coil 72 of the electromagnetic shutter 60 isdeenergized since it is connected to the battery in series with theswitch 70. This ends the magnetic force the coil 72 exerted on theparamagnetic plunger 74 allowing the shutter to be drawn upwardly by aspring 76 connected under tension between the shutter and a fixedsupport. This moves the aperture 58 upwardly, blocking the fiow of lightto the photosensitive resistive element 40. Photosensitive resistanceelements have a tendency to deteriorate when exposed to light and thisis to protect the photosensitive resistive element from light when it isnot performing its function.

The invention has now been described in its preferred embodiment. Itwill be understood that the present application is not limited to thepreferred embodiment of the invention but it is intended to cover allchanges and modifications of the preferred embodiment which do notconstitute departures from the spirit and scope of the invention.

What is claimed is:

1. A control circuit for the headlamps of an automobile powered directlyfrom the car carried battery and which will automatically dim the highbeams at the approach of oncoming motor vehicles and will automaticallyreturn the high beams when oncoming motor vehicles pass comprising aphotosensitive resistive element positioned in the automobile so thatlight from the headlamps of oncoming motor vehicles will strike thephotosensitive resistive element and decrease its resistance, anamplifying means having an input stage with a field effect transistorand an output terminal at which the magnitude of current flow will varywith changes in the magnitude of current flow through the field effecttransistor, biasing means including said photosensitive resistiveelement connected to the field effect transistor so as to vary themagnitude of current flow at the output terminals of the amplifyingmeans as motor vehicles approach and pass, and an electromagnetic relayhaving armature means connected to the battery of the automobile, firstcontact means connected to the low beam filaments of the headlamps ofthe automobile, second contact means connected to the high beamfilaments of the headlamps of the automobile and an electromagnetic coilconnected to the output terminals of the amplifying means so that therelay can be energized and deenergized by the changes in current fiow atthe output terminals of said amplifying means to position the armaturemeans either against the first contact mean or against the secondcontact means.

2. The headlamp control circuit of claim 1 including anelectromagnetically actuated shutter means which shields thephotosensitive resistive element from light when the automobilesheadlamps are off but which permits light to reach the photosensitiveresistive element when the automobiles headlamps are on.

3. The headlamp control circuit of claim 1 wherein said biasing circuitback biases said field effect transistor, said back bias being increasedby light impinging in the photo resistive element.

4. The headlamp control crcuit of claim 3 wherein said biasing circuitvaries as a function the position of the armature. to increase the backbias on the field effect transistor when the armature is against thefirst of the contacts.

5. The headlamp control circuit of claim 4 wherein said biasing circuitincludes said photosensitive resistive element connected between thegate terminal and source terminal of the field effect transistor, afirst resistance means connected between the gate terminal and the drainterminal of the field effect transistor and a second resistance meanswhich is coupled between the gate terminal and the drain terminal ofsaid field effect transistor only when the armature is positionedagainst the second contact means.

References Cited by the Examiner UNITED STATES PATENTS 2,760,114 8/1956Falge et a1. 250-214 2,917,666 12/1959 Engelmann et a1. 250214 RALPH G.NILSON, Primary Examiner.

M. ABRAMSON, Assistant Examiner.

1. A CONTROL CIRCUIT FOR THE HEADLAMPS OF AN AUTOMOBILE POWERED DIRECTLYFROM THE CAR CARRIED BATTERY AND WHICH WILL AUTOMATICALLY DIM THE HIGHBEAMS AT THE APPROACH OF ONCOMING MOTOR VEHICLES AND WILL AUTOMATICALLYRETURN THE HIGH BEAMS WHEN ONCOMING MOTOR VEHICLES PASS COMPRISING APHOTOSENSITIVE RESISTIVE ELEMENT POSITIONED IN THE AUTOMOBILE SO THATLIGHT FROM THE HEADLAMPS OF ONCOMING MOTOR VEHICLES WILL STRIKE THEPHOTOSENSITIVE RESISTIVE ELEMENT AND DECREASE ITS RESISTANCE, ANAMPLIFYING MEANS HAVING AN INPUT STAGE WITH A FIELD EFFECT TRANSISTORAND AN OUTPUT TERMINAL AT WHICH THE MAGNITUDE OF CURRENT FLOW WILL VARYWITH CHANGES IN THE MAGNITUDE OF CURRENT FLOW THROUGH THE FIELD EFFECTTRANSISTOR, BIASING MEANS INCLUDING SAID PHOTOSENSITIVE RESISTIVEELEMENT CONNECTED TO THE FIELD EFFECT TRANSISTOR SO AS TO VARY THEMAGNITUDE OF CURRENT FLOW AT THE OUTPUT TERMINALS OF THE AMPLIFYINGMEANS AS MOTOR VEHICLES APPROACH AND PASS, AND AN ELECTROMAGNETIC RELAYHAVING ARMATURE MEANS CONNECTED TO THE BATTERY OF THE AUTOMOBILE, FIRSTCONTACT MEANS CONNECTED TO THE LOW BEAM FILAMENTS OF THE HEADLAMPS OFTHE AUTOMOBILE, SECOND CONTACT MEANS CONNECTED TO THE HIGH BEAMFILAMENTS OF THE HEADLAMPS OF THE AUTOMOBILE AND AN ELECTROMAGNETIC COILCONNECTED TO THE OUTPUT TERMINALS OF THE AMPLIFYING MEANS SO THAT THERELAY CAN BE ENERGIZED AND DEENERGIZED BY THE CHANGES IN CURRENT FLOW ATTHE OUTPUT TERMINALS OF SAID AMPLIFYING MEANS TO POSITION THE ARMATUREMEANS EITHER AGAINST THE FIRST CONTACT MEANS OR AGAINST THE SECONDCONTACT MEANS.